1. Field of the Invention
This invention relates to X-ray spot film devices, and more particularly, to an improvement in an X-ray field defining mask which is used in conjunction with an X-ray spot film device of an X-ray table to make a selected number of radiographic exposures on a single film.
2. Description of the Prior Art
Conventional spot film devices usually include a frame support extending width-wise over the top of an X-ray table. A main carriage is mounted on the support to be advanced from a rear parked position to a forward radiographic position wherein a film holder carried by the carriage is disposed in alignment with an X-ray beam that is projected from an X-ray source in the table through a patient. The film holder is mounted in a tray supported on an inner carriage which is translatable widthwise of the main carriage so that the center of the X-ray beam may be made coincident with the area on the film holder on which a spot film exposure is desired. The area is further defined by superimposable X-ray field defining masks.
As is well known, spot film devices are also provided with a fluoroscopic device which permits an examining radiologist to visualize anatomy of interest and to make one or more radiographs in a choice of sizes by projecting the film holder forward and shifting it and the masks to obtain the desired sequence of radiographs. The fluoroscopic device on the spot film device is aligned with the X-ray source in the table, and the film cassette is, of course, retracted from the beam fluoroscopy. The film holder may be shifted relative to the masks to expose different areas of the film and to make exposures of different sizes.
In prior art spot film devices two X-ray field defining masks having elongated rectangular apertures disposed at right angles to each other are used. The masks are mounted for translating on a pair of tracks which are arranged in parallel near opposite margins of the support. These masks are translated by a respective mask drive system including a reversible servo motor. One of the masks is moved manually into the X-ray beam when two adjacent widthwise exposures are to be made on the film. The other mask is disposed in the beam when two adjacent exposures lengthwise of the film are desired. The masks are superimposed so that their rectangular apertures are widthwise. The film is positioned for exposing a quadrant or corner area of the film, and then the film may be shifted three more times to expose the remaining three quadrants.
The mask is made in a laminar fashion so it will have lightweight strength and yet have sufficient X-ray attenuation in the proper areas to cut off undesired portions of the X-ray beam and define a sharp field. The laminates are a stiff sheet such as stainless steel or brass on which a thinner lead sheet is bonded.
An example of the conventional spot film device is disclosed in U.S. Pat. No. 3,862,426 issued Jan. 21, 1975. Such masks have, however, certain undesirable characteristics. For example, the mask has greater external dimensions than the X-ray film or film holder. This requires the spot film device have a large space for a parked position to which the masks are retracked. The mask is made of stacked stiff sheet metals so that the power on the driving system for driving the mask through most of its travel is increased in proportion to the weight of the mask. Furthermore, the longitudinal edge of the aperture in the mask is flexual due to its low flexural rigidity. Attempts have been made to prevent collision of the longitudinal edge with the leading edge of the mask by spacing the tracks in which the masks slide in a direction perpendicular to the mask sliding plane. However, such a spacing of the spot film device decreases the quality of the X-ray image.